Electric arc welder and method for controlling the welding process of the welder

ABSTRACT

Electric arc welder ( 10 ) performs a given weld process with selected current waveform performed between electrode ( 24 ) and workpiece ( 26 ). Welder ( 10 ) includes a controller with a digital processor ( 30 ), sensor ( 44 ) for reading instantaneous weld current, and circuit ( 56 ) to convert the instantaneous current into a digital representation of the level of instantaneous weld current. Digital processor ( 30 ) has program circuit ( 64, 66 ) to periodically read and square the digital representation at a given rate, register ( 70, 72 ) for summing a number N of square digital representations to give a summed value, and an algorithm ( 82, 84, 86, 88 ) for periodically dividing the summed value by the number N to provide a quotient and then taking the square root of said quotient to thereby digitally construct rms signal ( 40 ) representing the root mean square of the weld current.

This application is a continuation of U.S. Ser. No. 10/603,322 filedJun. 26, 2003 which has since issued as U.S. Pat. No. 7,091,445 on Aug.15, 2006, which claims priority from U.S. provisional application Ser.No. 60/421,078 filed Oct. 25, 2002. Application Ser. Nos. 10/603,322 and60/421,078 are incorporated herein by reference in their entireties.

The present invention relates to the field of electric arc welding andmore particularly to a novel electric arc welder and a system and methodfor controlling the welding process performed by the welder.

INCORPORATION BY REFERENCE

The invention relates to an electric arc welder for performing a weldingprocess between an electrode and a workpiece wherein the welding processis comprised of a succession of current waveforms. Such currentwaveforms are created by a number of individual current pulses occurringat a frequency of at least 18 kHz with a magnitude of each of thecurrent pulses being controlled by a wave shaper or waveform generator.In this type of electric arc welder, the waveform generator or waveshaper digitally controls a digital pulse width modulator, usually aprogram in the controller DSP. The pulse width modulator controls theswitching of a high speed switching type power source, such as aninverter. This waveform control technology implemented in an electricarc welder has been pioneered by The Lincoln Electric Company ofCleveland, Ohio and is generally disclosed in Blankenship U.S. Pat. No.5,278,390. The Blankenship patent is incorporated by reference herein asbackground illustrating a high speed switching power source, such as ainverter, for controlling a weld process including a series ofcontrolled waveforms determined by the output of a waveform generator orwave shaper.

The invention involves an embedded algorithm for obtaining the root meansquare of either the welding current or the welding voltage, as well asaverage power. The concept of an embedded system programming of the typeused in the present invention is generally disclosed in an article byJack W. Crinshaw entitled Embedded Systems Programming (Integer SquareRoot) This article published in February 1998 is incorporated byreference herein as illustrating the background technology used in thedigital signal programmer of a standard controller associated with anelectric arc welder. Also incorporated by reference herein is an articleentitled Electrical Measurements and Heat Input Calculations for GMAW-PProcess dated Nov. 2001. This article teaches away from the presentinvention and suggests that electric arc welding should be controlled bythe average current using a slow sample rate no greater than 4.5 kHz.This article is incorporated by reference illustrating the advantage ofthe use of the present invention over prior suggestions.

BACKGROUND OF THE INVENTION

As illustrated in prior patents and literature, electric arc welding hasheretofore used the average weld voltage and the average weld currentfor controlling the operation of the power source in the welder. Thedigital controller includes a digital signal processor (DSP) forcontrolling a waveform generator or wave shaper that directs theoperation of the normal pulse width modulator. This device creates thewaveforms successively used by the welder to perform the weldingprocess. Welders regulate the output current or voltage to an averagevalue such as an average weld current by a feedback loop. For a constantvoltage process that is welding in the “spray” region, the averagecurrent is an accurate gage of the welding process. However, in pulsewelding, the average current and average voltage do not accuratelyreflect the result of the welding process including the deposition rate,heat zone and penetration. This is explained by a example of an idealpulse welding process, such as one having 500 amperes for 25% of thetime and 100 amperes of background current for 75% of the time has anoutput current of 200 amperes. However, the average current of thewelding process merely indicates the deposition rate and does notreflect the true heat input to the welding operation. Consequently, whenthe welding process is controlled by a series of repetitive waveforms,such as A.C. welding or pulse welding, average current values can notcontrol the heat input. Recently, the welding processes have becomequite complex and now often involve a number of successive waveforms,such as A.C. current and pulse current, so the old technology offeedback control for the welding process is not completely accurate andrequires a substantial amount of on-site manipulation by a personknowledgeable in welding, especially a person knowledgeable in the newwaveform welding procedure using a welder, such as shown in BlankenshipU.S. Pat. No. 5,278,390. With the advent of pulse welding using waveformgenerators and high speed switching power sources, such as inverters,the obtained weld heat has been adjusted by trial and error. Too muchheat causes metal to burn through, especially in thin metal welding.Thus, the welding engineer modulates the average current and averagevoltage to provide the heat input to the welding process to a level sothat burn through is theoretically eliminated. This procedure wasapplicable, however, only for a pure spray type welding process. Thisprocedure of controlling the heat by the average current and averagevoltage was not applicable to the new generation of electric arc welderswhere waveforms are changed to control the welding process. This is thenew waveform control technology to which the present invention isdirected. The old technology used for non-waveform welding isinapplicable to controlling heat in a controlled waveform type welder.The heat is not known by merely reading the voltage and current when thenew waveform type arc welders are employed. Consequently, the weldingengineer when using waveform control technology changed the basefrequency during pulse welding while maintaining a constant or setaverage voltage. Using this approach of frequency adjustment of a pulsewelding procedure while maintaining a constant voltage, the heat couldbe adjusted by a trial and error technique. When this trial and errorprocedure was used to modify the waveforms in a new waveform welder, theheat could, indeed, be controlled; however, it was not precise andinvolves substantial technical knowledge combined with the trial anderror procedures.

There is a distinct advantage in pulse welding. This welding processlowers the heat into the joint for the same wire feed speed as a “spray”or “globular” weld process. Thus, a lower heat setting can be set at thefactory. The welder had a knob to adjust the nominal frequency, for thepurpose indicated above. This change in base frequency did adjust theheat at the welding operation. This resulted in a slight change in thepower factor of the welding process through the trial and error methodwhen knowing that the average voltage times average current multipliedby the power factor equals the input heat. Thus, by using a knob tochange the base frequency, the power factor was changed to determineheat. However, neither the factory nor the welding engineer at thewelding site had the capabilities of directly controlling the powerfactor. Computation of actual power factor on the fly was not realizedin prior control systems and method used for electric arc welders evenof the type that used a waveform or wave shape control of the weldingprocess. Consequently, with the introduction of the new waveform weldingpioneered by The Lincoln Electric Company, there is a need to controlthe welding parameters to a value that accurately reflects the heatcontent. Only in this manner can weld parameters be used in a closedloop feedback system, or otherwise, to control the penetration and heatseparately in a weld process using generated waveforms.

SUMMARY OF THE INVENTION

With the advent of the new wave shapes developed for electric arcwelding, the present invention provides a control of the weldingparameters to accurately reflect the heating content without use oftrial and error procedures or the need for on site welding engineers tomodulate and control the welding process. The invention is in weldingwith a series of generated waveforms, such as A.C. welding or pulsewelding.

In order to produce a stable weld while continuously feeding wire intothe weld puddle, there are primarily two factors that must be balanced.First, the amount of weld metal wire and its material propertiesdetermine how much current is needed to melt the wire. Second, theamount of heat determines the heat affected zone or penetration of thewelding process. In the past, an operator dialed in a voltage and wirefeed speed and manually adjusted the electric stickout to control theamount of heat put into the weld. Welding literature typically claimsthat the pulse welding process lowers the current for the samedeposition rate of a “spray” procedure. This is technically accurate.The average current is, indeed, much less than the average current of anequivalent “spray” procedure when using “pulse” welding. However, therms currents of both procedures are about the same. The presentinvention involves the use of rms current for the feedback loop controlof the welding process. Thus, the invention involves the use of rmscurrent and rms voltage for controlling the welding process, especiallywhen using a series of generated pulse waves, such as in A.C. weldingand “pulse” welding using the technology described in Blankenship U.S.Pat. No. 5,278,390. By using the rms current and rms voltage, a moreaccurate control of the waveform type welding process is maintained. Inaccordance with the invention, the rms value and the average value ofcurrent and voltage can be used for feedback control. In this aspect ofthe invention, a first constant is multiplied by the rms value and asecond constant is multiplied by the average value of the parameter.These two constants total one, so the constituent of root mean square inthe feedback control is adjusted with respect to the constituent ofaverage in the feedback control. These constants preferably total one.In practice, the rms constant is substantially greater than the averagevalue constant so that normally the rms value is predominate over theaverage value. It has been found that the rms value more accuratelyreflects the heating value of the welding process.

In accordance with a primary aspect of the invention, the feedbackcontrol of the electric arc welder maintains the rms voltage and rmscurrents constant, while adjusting the calculated real time powerfactor. This procedure of adjusting the power factor adjusts the heatinput to the weld procedure to a desired level.

Since the rms values of current and voltage are obtained by using thepresent invention, the power factor can be obtained by calculationswithin an embedded algorithm in the digital signal processor of theelectric arc welder. By maintaining the power factor constant, theelectric arc welder will automatically adjust for tolerances experiencedin cable impedance at the welding site. By maintaining the power factorconstant by using the present invention, the control for the weldingprocess of the electric arc welder can be set at the manufacturingfacility by setting the power factor at a given value and the rmscurrent at a given value. When the welder is then installed at themanufacturing facility, the rms current is maintained at the desiredconstant set at the factory and the power factor is maintained at theset value. The power factor is, thus, accurately controlled at the site.By use of the present invention irrespective of changes in the cableimpedance, the welding process will be the same.

In the present invention the term “power factor” relates to the powerfactor of the welding process. This is a parameter obtained by using thepresent invention through the digital signal processor (DSP) of a welderhaving an embedded algorithm for calculating the root mean square ofboth current and voltage. By using the present invention, the powerfactor is held constant by a closed loop feedback. This feedback systemkeeps the heat of the welding process at a given selected and desiredlevel.

Since the present invention involves the determination of a real timerms value for current arid/or voltage, a closed loop feedback maintainsthe rms current at a constant desired level. The actual power factor isalso generated for a closed loop feedback system so that the weldingpower factor is adjusted to change the average power and, thus, the heatof the welding operation. Consequently, another aspect of the inventionis maintaining the rms current constant while adjusting the power factorto change the heat at the welding process. When this is done in awaveform type welder wherein the waveform is created by a number ofcurrent pulses occurring at a frequency of at least 18 kHz with amagnitude of each pulse controlled by a wave shaper, the shape of thewaveform in the welding process is modified to adjust the power factor.In this aspect of the invention, the current remains constant. Thiscould not be accomplished in other types of welders, nor in waveformcontrol welders, without use of the present invention.

The present invention relates to a control of an electric arc welder ofthe type wherein a pulse width modulator, normally in the DSP, controlsthe current waveform constituting the welding process. By using thepresent invention, the rms current and rms voltage is obtained for thepurpose of combining with the average current and average voltage toproduce, not only the average power, but also the actual real time powerfactor. Consequently, the actual power factor can be adjusted, theactual rms current can be adjusted, or the actual rms voltage can beadjusted. In all of these embodiments, the adjustment of the constructedor calculated parameters modifies the waveform to control the weldingprocess accurately in the areas of penetration and heat input. By havingthe capabilities of the present invention, power factor manipulationadjusts the heat input of the welding process. In accordance with anaspect of the invention, the feedback of current and voltage is acombination of the rms value and the average value in a method or systemwhere the rms value predominates.

In pulse welding, the rms current and/or the average current is adjustedto maintain a constant rms voltage. When using the invention for A.C.welding, the output of the welder is regulated to provide a constant rmsor a constant rms current. Turning now to a MIG welding process, theoutput of the machine is regulated to maintain a constant rms current.The basic aspect of the invention is creating a rms current value on areal time basis and/or a rms voltage value on a real time basis. Theseparameters are regulated by changing the wave shape of the weldingprocess utilizing a standard closed loop feedback system or method. Byhaving the capabilities of creating a rms current, power factor iscalculated by the digital signal processor of the arc welder for use ineither maintaining a constant power factor or adjustment of the powerfactor to control heat in a pulse welding process.

The invention is primarily applicable for use in an electric arc welderof the type having a pulse shaper or waveform generator to control theshape of the waveform in the welding process. This type of welder has adigitized internal program functioning as a pulse width modulatorwherein the current waveform is controlled by the waveform generator orwave shaper as a series of current pulses. The duty cycle of these highspeed pulses determines the magnitude of the current at any givenposition in the constructed waveform of the weld process. This type ofwelder has a high speed switching power source, such as an inverter. Theinvention involves the combination of this particular type of powersource and implementation of the program and algorithm to form thefunctions set forth above.

In accordance with the invention, there is provided an electric arcwelder for performing a given weld process with a selected waveformperformed between an electrode and a workpiece. This type of weldergenerates the waveforms and includes a controller with a digital signalprocessor. The sensor reads the instantaneous weld current and a circuitconverts the instantaneous current into a digital representation of thelevel of the instantaneous current. The digital processor has a programcircuit or other program routine to periodically read and square thedigital representation at a given rate. A register in the processor sumsa number of squared digital representations to create a summed value. Anembedded algorithm in the processor periodically divides the summedvalue by a number N, which is the number of samples obtained during thesampling process of the waveform. The quotient provided by dividing thesummed value by the number of samples is then directed to the algorithmfor taking the square root of the quotient to thereby digitallyconstruct an rms signal representing the root mean square of the weldcurrent. This same procedure is used for obtaining the root mean squareor rms signal representing the weld voltage. Consequently, the initialaspect of the invention is the use in a waveform welder, a real timesignal indicative of the root mean square of the weld current primarily,but also the weld voltage. These signals have not heretofore beenobtainable in an arc welder of the type to which the present inventionis directed.

In accordance with another aspect of the invention, the controller ofthe specific type of waveform welder defined above includes a feedbackcontrol loop with an error detector for generating a current controlsignal based upon the relationship of two signals. This is a standardclosed loop system or method. In performing this system or method, thefirst of the two signals includes the root mean signal obtained by thefirst aspect of the present invention. The term “included” is used inthis aspect of the invention since a modification of the invention has afeedback using a combination of the root mean value of current orvoltage plus a component of the average current and voltage. However, inaccordance with an aspect of the invention, the rms value predominates,since it has preferred control characteristics. The use of a feedbackwith a component of the average current or voltage is a more limitedaspect of the present invention. In an aspect of the invention, thecontributing component of the root mean square and contributingcomponent of the average is equal to one. The root mean squarecontribution predominates in accordance with an aspect of the invention.

As previously stated, the present invention is directed to an electricarc welder of a specific type wherein a waveform is generated by awaveform generator or wave shaper. Consequently, another aspect of thepresent invention is the provision of an electric arc welder as definedabove wherein the waveform is created by a number of current pulsesoccurring at a frequency of at least 18 kHz, with a magnitude of eachpulse controlled by a wave shaper or waveform generator. The “switchingfrequency” is the frequency of the pulse width modulator controlling theswitching frequency of the power source. This frequency is normallysubstantially greater than 18 kHz and preferably in the range of 40 kHz.

The invention, as defined above, has a sampling rate for the sensedcurrent and/or voltage. In accordance with another aspect of the presentinvention, this sampling rate is less than 40 kHz or in another aspectit is in the general range of 5 kHz to 100 kHz. In practice, thesampling rate provides a sample each 0.10 ms. It is anticipated thatthis rate should have a time as low as 0.025 ms.

In accordance with an aspect of the invention, there are first andsecond registers, or buffers, for summing the squared digitalrepresentations of either current or voltage. A circuit is controlled bythe waveform to generate an event signal T at a given location in thewaveform. This event signal T stops the sampling and determines thenumber N of samples taken. This number N is used in processing eitherthe average current or the rms current. By using two registers, orbuffers, the squared value is loaded during each sample of the waveformuntil the circuit determining the end of the waveform is activated toprovide an event signal T. Thus, the present invention utilizes thenumber of samples N and the event signal T to determine the parametersof a given current waveform. These waveforms are repeated andsuccessively analyzed. At the end of the waveform, as announced by theexistence of an event signal T, the digital signal processor utilizesits background time to calculate the rms values and other aspects of theinvention as the digital signal processor is performing the normalwelding control features of the welder. The algorithm calculations isinitiated by the existence or creation of the event signal T. The samplenumber N is obtained by a digital counter that counts at the sample rateand is reset and read at the signal T.

In accordance with still a further aspect of the present invention,there is provided an electric arc welder for performing a given weldprocess with a selected current waveform performed between an electrodeand a workpiece. This welder comprises a controller with a digitalsignal processor. In accordance with standard technology, the controllerhas a waveform generator for creating a number of different currentpulses occurring at a frequency of at least 18 kHz with a magnitude foreach pulse creating the waveform and a feedback current control loopwith and error detector program for generating a current control signalbased upon the relationship of two input signals. This is essentiallywaveform technology pioneered by The Lincoln Electric Company. Inaccordance with the invention, the first input signal includes a rmscurrent signal generated by an algorithm processed by the digitalprocessor. The second signal is a signal representing the desiredcurrent or rms signal. Consequently, the desired rms signal selected bythe welder manufacturer or adjusted by a welding engineer is tracked bythe actual rms current signal of the welder. In this manner, the rmscurrent is held constant. When that occurs, tolerance in the on sitecables and other process affecting environmental features are ignored inthe welder control scheme. This concept is best used with a calculationof the power factor. Using another aspect of the present invention, thepower factor created by using the present invention and the rms currentare both maintained constant. Thus, when the welder is programmed at themanufacturing facility with a set rms current and set power factor,these parameters are maintained at the welding operation. Consequently,the welding process at the welding operation duplicates the weldingprocess set into the welder by the welding engineer at the manufacturerof the welder.

Still a further aspect of the present invention is the provision of amethod of operating an electric arc welder for performing a given weldprocess with a selected current waveform performed between an electrodeand a workpiece. This method involves the process procedure set forthabove, wherein the current is sensed and the root mean square currentvalue is calculated as a signal to be used for feedback control of theelectric arc welder. This same method is used for obtaining a rmsvoltage signal used in controlling the electric arc welder of thepresent invention. Still another aspect of the present invention is theprovision of an electric arc welder for performing a given weldingprocess, as defined above. The welder includes a circuit to sample theweld current of the waveform at a give rate, a detector for creating anevent signal at a given location in the waveform, a counter to count thenumber N of samples in successive event signals, a digital processorwith an algorithm to calculate a rms value for the weld current basedupon the samples taken and the number N counted. The number N and theevent signal T have been described in connection with another aspect ofthe present invention. The invention is used in the sample concept andevent signal concept to create values used in an algorithm of acontroller for a specific type of electric arc welder. In this manner,the rms value is used to change the waveform to adjust the operation ofthe waveform welder. The frequency of the switching power source forcreating the waveform has an oscillator with a frequency exceeding 18kHz. Another aspect of the invention is the method of operating theelectric arc welder as defined in this aspect of the invention.

Another basic aspect of the present invention is the provision of anelectric arc welder for performing a given welding process with aselected waveform, as defined above. In this aspect of the invention,the welder comprises a power source with a controller having a digitalprocessor including a program to calculate the real time power factor ofthe weld current and the weld voltage. The program of this aspect of theinvention includes an algorithm to calculate the rms weld voltage, therms weld current and the average power of the power source. The averagepower is obtained by using a calculated value for the average currentand the average voltage. These average values are obtained by the samesampling arrangement described in accordance with the rms portion of thepresent invention. The sampling and the number of samples is stopped atan event signal T. Then the current and/or voltage digitized values areadded and divided by the number of samples. This provides the averagecurrent and average voltage. By multiplying this values together the“average power” is obtained. The concept of obtaining an average powerfor an electric arc welder is not novel. However, the process ofobtaining the average power utilizing the sampling concept and the eventsignal T is novel in a welder of the type using waveforms thatconstitute the welding process.

In an aspect of the invention, the average power is obtained togetherwith the rms current and the rms voltage. A circuit divides the averagepower by the rms power to create a signal or level representing theactual real time power factor of the power source. This power factor iscompared with the desired power factor to create a corrective value forthe wave shaper whereby the actual real time power factor is held at thedesired power factor. This maintains a constant power factor. Asexplained before, by maintaining a constant power factor with a constantrms current, any tolerances in the welding process are overcome so thatthe welder will operate identically at the site as it did when set up bythe manufacturer. This aspect of the invention is primarily employed forpulse welding and changes the shape of the pulse to obtain the desiredconstant power factor without changing the root mean square current ofthe welding process.

In accordance with another aspect of the invention relating to theobtained power factor level, the power factor is adjustable to changethe heat of the welding process, especially when using the invention forpulse welding. The waveform generator or wave shaper controls the shapeof the waveform to adjust the power factor to maintain it constant or toadjust it for the purposes of controlling heat. When this adjustment isemployed, the rms current is maintained constant. Thus, the power factoris adjusted without adjusting or changing the actual current. The rmscurrent determines the melting rate of the metal.

In accordance with another aspect of the present invention there isprovided a method of controlling an electric arc welder, of the typedefined above, which method comprises calculating the actual powerfactor of the power source using the rms current and the rms voltage. Adesired power factor is then selected for the power source and an errorsignal is obtained by comparing the actual power factor of the powersource to the desired power factor of the power source. This isaccomplished by adjusting the waveform by the error signal whereby theactual power factor is held at the desired power factor.

The primary object of the present invention is the provision of anelectric arc welder of the type using a waveform generator or waveshaper whereby the rms current and/or rms voltage is obtained forcontrolling the welding process.

Still a further object of the present invention is the provision of awelder as defined above which welder is operated in accordance with amethod that calculates the rms current and/or the rms voltage and, fromthis calculation obtains the actual power factor at the welding process.The power factor value controls the heat or maintains a constant powerfactor for the weld process.

Still a further object of the present invention is the provision of anelectric arc welder and method of operating the same, which welder andmethod utilizes existing waveform technology and the common digitalsignal processor of the welder to calculate rms values and use thesevalues for determining the power factor of the welding process so thatthese parameters are used in the feedback control systems of theelectric arc welder.

These and other objects and advantages will become apparent from thefollowing description.

BRIEF DESCRIPTION OF DRAWINGS

The invention is apparent from the drawings which are:

FIG. 1 is a block diagram illustrating an electric arc welder utilizingthe present invention for controlling the waveform generator;

FIG. 2 is a flow chart and block diagram illustrating the computerprogram of the digital signal processor utilized for performing thepreferred embodiment of the present invention;

FIG. 2A is a cycle chart of digital signal processor utilized forperforming the preferred embodiment of the present invention as setforth in FIG. 2 showing the timing function of the digital signalprocessor;

FIG. 3 is a flow chart of the program for implementing aspects of thecycles in FIG. 2A after creation of an event signal T;

FIG. 3A is a waveform graph for the logic applied to the state table inFIG. 3;

FIG. 4 is a current waveform graph illustrating the sampling conceptused in the present invention to create current signals used inobtaining rms values;

FIG. 5 is a block diagram and flow chart of the cycle counter in a fieldprogrammable gate array incorporated in the controller and a blockdiagram of the use of this cycle counter information in the digitalsignal processor (DSP) to obtain an event signal T;

FIG. 5A is a graph of the pulse current and logic at one terminal of theflow chart shown in FIG. 5 when pulse welding is used instead of A.C.welding;

FIG. 6 is a flow chart of the preferred embodiment of the presentinvention as performed in the digital signal processor during the cyclesshow in FIG. 2A;

FIG. 7 is a block diagram of the program used to create the rms currentsignal using the present invention;

FIG. 8 is a block diagram like FIG. 7 for creating the rms voltagesignal;

FIG. 9 is a block diagram showing the aspect of the invention forcreating an average power signal;

FIG. 10 is a block diagram showing the aspect of the present inventionfor creating the actual power factor of the welding process for use inthe present invention;

FIG. 11 is a block diagram of a welder utilizing the power factor valueof FIG. 10 to maintain a constant power factor for the weld process inpulse welding;

FIG. 12 is a block diagram, as shown in FIG. 11, wherein the powerfactor value from FIG. 10 is adjusted manually to control the powerfactor of the welding process while maintaining the rms currentconstant;

FIG. 13 is a block diagram showing a standard digital filter controlledby the relationship of the actual power factor to the set power factorto adjust the shape of the weld current by adjusting the waveformgenerator input to maintain a constant power factor;

FIG. 14 is a block diagram showing control of the welder by arelationship of average voltage and a rms voltage compared with a setvoltage signal to adjust the shape of the waveform to maintain a setvoltage;

FIG. 15 is a block diagram showing control of the welder by arelationship of average current and a rms current compared with a setcurrent signal to adjust the shape of the waveform to maintain a setcurrent;

FIG. 15A is a current graph showing how the waveform is adjusted tomaintain the set value, be it current, voltage or power factor; and,

FIG. 16 is a block diagram showing a digital filter to adjust the wirefeed speed based upon a comparison of a set voltage to a signalinvolving a component of average and rms voltage and also a digitalfilter to adjust the waveform upon a comparison of a set current to asignal involving a component of average and rms current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, electric arc welder 10 is shown in blockdiagram form. A three-phase rectifier 12 provides power to high speedswitching-type power supply 14 across a DC link in the form of inputleads 16, 18. In a preferred embodiment, high speed switching-type powersupply 14 is an inverter, such as a Power Wave welding power supplyavailable from Lincoln Electric Company of Cleveland, Ohio. However, ahigh speed switching chopper or other high speed switching-type powersupply can also be employed. High speed switching-type power supply 14performs a preselected welding process. In accordance with presentwelding technology, high speed switching-type power supply 14 preferablyswitches at about 18 kHz or higher, and more preferably at 40 kHz orhigher. High speed switching-type power supply 14 energizes weldingcircuit 20 that includes inductor 22 and electrode 24 forming an arc gapwith workpiece 26 during performance of the welding operation.Typically, electrode 24 is a forward advancing welding wire from asupply spool. The welding wire is driven toward workpiece 26 at aselected wire speed during performance of the welding operation.

Controller 30 controls high speed switching-type power supply 14 duringthe welding operation. In accordance with the present weldingtechnology, controller 30 is a digital device including waveformgenerator 32 that outputs power level waveform 34 represented by a linethat is the input to pulse width modulator 36. Pulse width modulator 36produces pulse train 38 (represented by a line) of pulses with pulsewidths corresponding to the power level of waveform 34. In other words,waveform 34 is converted into pulse width modulated pulse train signal38 by pulse width modulator 36. Pulse width modulator 36 produces pulsesof controlled width at a frequency preferably above 18 kHz, and morepreferably about 40 kHz or higher, which is the input to high speedswitching-type power supply 14. The power supply switching is controlledby pulse -width modulated pulse train 38 to energize welding circuit 20in accordance with power level waveform 34.

Waveform 34 implements a desired welding process. Typically, a weldingprocess is made up of a waveform train of repeating waveforms. For pulsewelding, power level waveform 34 has a preselected wave shape forgenerating a welding process pulse. The average power or true heatproduced in the welding process implemented by waveform 34 over a timeinterval [T₁, T₂] is given by:

$\begin{matrix}{{P_{avg} = {\frac{1}{T_{2} - T_{1}}{\int_{T_{1}}^{T_{2}}{{{v(t)} \cdot {i(t)}}\ {\mathbb{d}t}}}}},} & (1)\end{matrix}$where P_(avg) is the average power, v(t) is the instantaneous voltage,i(t) is the instantaneous welding current, v(t)·i(t) is theinstantaneous power, and T₁ and T₂ are the starting and ending timepoints of the time interval, respectively, of the integration. In thecase of a substantially periodic waveform, the average power can beexpressed in terms of root-mean-square (rms) voltage and rms currentaccording to:P _(avg) =V _(rms) ·I _(rms) ·PF  (2),where the rms voltage, V_(rms), and rms current, I_(rms), are given by:

$\begin{matrix}{{V_{rms} = \sqrt{\frac{\int_{T_{1}}^{T_{2}}{\left\lbrack {v(t)} \right\rbrack^{2}\ {\mathbb{d}t}}}{T_{2} - T_{1}}}},{I_{rms} = \sqrt{\frac{{\int_{T_{1}}^{T_{2}}\left\lbrack {i(t)} \right\rbrack^{2}}\ }{T_{2} - T_{1}}}},} & (3)\end{matrix}$respectively, and PF is the power factor. In computing the average powerand the rms current and voltage values for waveform 34 that implementspulse welding, the time interval [T₁, T₂] preferably corresponds to onepulse or a plurality of pulses. In waveform-controlled welding, thepulse time interval may vary for successive pulses. Hence, in thedescribed preferred embodiment, the starting and stopping times T₁ andT₂ are extracted from waveform 34 as event signals T determined from acharacteristic feature of waveform 34.

Equation (3) can be rewritten to define the power factor PF accordingto:

$\begin{matrix}{{PF} = {\frac{P_{avg}}{V_{rms} \cdot I_{rms}}.}} & (4)\end{matrix}$There is in general a close relationship for substantially any waveform34 between the rms voltage and current values and the average power.

In contrast, the average voltage, V_(avg), and average current, I_(avg),given by:

$\begin{matrix}{{V_{avg} = \frac{\int_{T_{1}}^{T_{2}}{{v(t)}\ {\mathbb{d}t}}}{T_{2} - T_{1}}},{I_{avg} = \frac{\int_{T_{1}}^{T_{2}}{{i(t)}\ {\mathbb{d}t}}}{T_{2} - T_{1}}},} & (5)\end{matrix}$have a close relationship with the average power only for certainwaveforms, such as are used in constant-voltage “spray” type welding.However, if, for example, the waveform includes a stepped pulse that is500 amperes for 25% of the time and 100 amperes for 75% of the time, therms value is 265 amperes, while the average value is 200 amperes. Inthis case, the rms values provide a more accurate true heat value.

With continuing reference to FIG. 1, controller 30 of electric arcwelder 10 implements an exemplary pulse welding process in which themagnitude of waveform 34 is controlled using an rms current 40 that iscalculated from an instantaneous welding current I_(a) 42 measuredacross shunt 44. In the constant current welding process shown in FIG.1, rms current 40 is compared with set rms current 46 by digital erroramplifier 48 to produce error signal 50 that controls an amplitude ofwaveform 34 to maintain a constant rms current. Similarly, for aconstant voltage welding process, control is suitably based on an rmsvoltage calculated from instantaneous welding voltage V_(a) 52 measuredacross the weld by voltmeter 54.

With reference to FIG. 2, computation of the rms current frominstantaneous welding current I_(a) 42 includes processing withanalog-to-digital converter 56 to produce digitized instantaneouscurrent 58, which is the input to digital signal processing block 60.Digital signal processing block 60 performs the current squaredintegration of Equation (3) digitally as a Riemann sum, dividing thecurrent into time intervals Δt defined by oscillator 62 for the summing.The digitizing interval Δt for the Riemann sum is suitably about 0.1milliseconds to provide adequate samples for each pulse or repetition ofwaveform 34. Sample-and-hold circuit 64 holds the digitized current forthe period Δt, and squaring processor 66 computes the square of the heldcurrent value.

In order to enable continuous summation of rms current in parallel withrelated processing such as the computation of the square-root operationof Equation (3), the summing preferably employs two alternating storagebuffers, namely first buffer 70 (identified as Buffer A), and secondbuffer 72 (identified as Buffer B). Values are stored in the activebuffer at intervals 76, 78 that are preferably in a range of about0.025-0.100 milliseconds. When first buffer 70 is active, switch 80transfers values at time intervals Δt to first buffer 70, whichaccumulates the current-squared values and also maintains a sample countN of a number of accumulated current samples. As a background processduring accumulation in first buffer 70, the contents of second buffer 72are processed by division processor 82 to divide by the number ofsamples N, and by square-root processor 84 to complete computation ofthe root-mean-square calculation of Equation (3).

At a selected event signal T generated by a characteristic of waveform34, the operation of buffers 70, 72 switches. Second accumulator 72 iscleared, and switch 80 subsequently transfers current-squared samplesinto second accumulator 72. As a background process during accumulationin second buffer 72, the contents of first buffer 70 are processed bydivision processor 86 to divide by the number of samples N, and bysquare-root processor 88 to complete computation of the root-mean-squarecalculation of Equation (3).

FIG. 7 shows a simplified block diagram of digital signal processingblock 60, which omits the details of the alternating summation buffers70, 72 and related switching circuitry that are shown in detail in FIG.2. In FIG. 7, current-squaring block 66, switch 80, and alternatingsummation blocks 70,72 are represented by a single summation block 100that sums current-squared samples between occurrences of the eventsignal T triggered by a characteristic of waveform 34, and alsomaintains the count N of the number of accumulated samples. Divisionbackground processes 82, 86 of FIG. 2 are represented by a singlenormalization background process 102 in FIG. 7. Square-root backgroundprocesses 84, 88 of FIG. 2 are represented by a single square rootbackground process 104 in FIG. 7.

With reference to FIG. 8, it will be appreciated that digital signalprocessing block 60 shown in FIG. 2 and represented in simplified formin FIG. 7 is readily adapted to perform rms voltage calculations, byreplacing measured instantaneous current I_(a) 42 with instantaneousvoltage V_(a) 52 measured by voltmeter 54 of FIG. 1. FIG. 8 shows rmsvoltage digital signal processing block 60′ in a simplified formanalogous to the simplified form of FIG. 7. The digitized voltage isprocessed by sample-and-hold circuit 64′ which holds the digitizedvoltage for the period Δt. Voltage-squared summation block 100′ sumsvoltage-squared samples and maintains a count N of the number ofaccumulated voltage samples. Preferably, summation block 100′ usesalternating summation buffers analogous to buffers 70, 72 shown for thecurrent-squared summation in FIG. 2. Normalization background process102′ divides the voltage-squared sample sum by the number of samples N.Square root background process 104′ takes the square root to completeimplementation of the rms voltage V_(rms) mathematically shown inEquation (3).

With reference to FIG. 9, it will be appreciated that digital signalprocessing block 60 shown in FIG. 2 and represented in simplified formin FIG. 7 is similarly readily adapted to perform average powercalculations, by inputting both measured instantaneous current I_(a) 42and measured instantaneous voltage V_(a) 52. FIG. 9 shows average powerdigital signal processing block 60″ in a simplified form analogous tothe simplified form of FIG. 7. Sample-and-hold circuits 64, 64′ whichhold the digitized current and voltage, respectively, for the period Δt,are accessed by current-times-voltage summation block 100″ which sumscurrent-times-voltage samples and maintains a count N of the number ofaccumulated current-times-voltage samples. Preferably, summation block100″ uses alternating summation buffers analogous to buffers 70, 72shown for the current-squared summation in FIG. 2. Normalizationbackground process 102″ divides the current-times-voltage sample sum bythe number of samples N to produce the average power P_(avg) shownmathematically in Equation (1).

Digital signal processing blocks 60, 60′, 60″ compute the rms current,the rms voltage, and the average power as Riemann sums. FIG. 4 showsexemplary current waveform 120 that is digitally sampled. Each digitalsample is represented by a rectangular sample bar 122 of time durationΔt and height corresponding to the digitized value of current waveform120 held by sample-and-hold circuit 64 at the time interval Δt.

Digital signal processing blocks 60, 60′, 60″ are optionally implementedas separate processing pathways that execute in parallel. However, in apreferred embodiment digital signal processing blocks 60, 60′, 60″ usesome common digital signal processing blocks into which the sampledvoltage and current signals are time-domain multiplexed. Such amultiplexing approach reduces the amount of circuitry required. Eachsummation (voltage-squared, current-squared, and voltage-times-current)has its own alternating summation buffer set (for example, summationbuffer set 70, 72 for summing current-squared values as shown in FIG.2).

With reference to FIG. 2A, a suitable process cycling for thetime-domain multiplexing is shown. The process cycling employs fourcycles 130, 132, 134, 136 each occupying one-fourth of the samplingperiod Δt. For the exemplary Δt equal 0.1 millisecond, each of the fourcycles 130, 132, 134, 136 occupies 0.025 milliseconds, During firstcycle 130, the voltage V_(a) and current I_(a) are digitized andsample/held. During second cycle 132, the current-squared is computedand added to the current-squared summation. During third cycle 134, thevoltage-squared is computed and added to the voltage-squared summation.During fourth cycle 136, a check is performed to determine whether anevent signal T has been detected, and the sample count is incremented.Moreover, throughout the cycling other processing, such as computationof the square roots of values stored in the inactive summation buffers,can be performed as background processes. Similarly, digital signalprocessing welding control operations, such as waveform shapingdescribed by Blankenship U.S. Pat. No. 5,278,390, can be performed asbackground control processes during the cycling.

With reference to FIGS. 2 and 2A, and with further reference to FIG. 3Aand FIG. 6, the cycling as applied to the current-squared calculation isdescribed. FIG. 3A illustrates current waveform 34 extending betweenfirst event signal T₁ and second event signal T₂. Event signals T₁, T₂are suitably generated by a circuit controlled by waveform 34. In FIG.3A, the circuit generates event signal T₁ responsive to onset of therising edge of current pulse 140, and the circuit generates event signalT₂ responsive to onset of the rising edge of current pulse 142. Thus,there is a current pulse between each two successive event signals T.Rather than detecting the rising edge, the event signals can instead begenerated by detecting another characteristic of the pulse, such as thefalling edge of the current pulse.

During the time interval between event signal T₁ and event signal T₂,current-squared samples are accumulated in summation buffer 70, asindicated in FIG. 3A by the notation “Adding to Buffer A”. Eachoccurrence of second cycle 132 of FIG. 2A adds another current-squaredsample to buffer 70. Although not shown in FIGS. 2, 3A, or 6,voltage-squared samples and average power samples are preferably beingaccumulated in their respective buffers during the other cycles of thefour-cycle process of FIG. 2A. Detection of event signal T₂ is indicatedby detection block 150 of FIG. 6. Responsive to detection 150, buffers70, 72 are switched so that buffer 72 is used to accumulatecurrent-squared samples of next pulse 142 of waveform 34, while buffer70 in which the current-squared samples of pulse 140 are accumulated isshifted 152 into the background. In background processing, thecurrent-squared sum is divided 154 by the number of samples N and thesquare-root is taken 156 to complete the rms algorithm. The computed rmscurrent value for pulse 140 is written 158 to a register for use inwelding process control.

With reference to FIG. 5, a suitable method for generating event signalsT is described. A field programmable gate array (FPGA) includes cyclecounter state machine 170 that updates two-bit counter 172. Statemachine 170 is configured to increment two-bit counter 172 each time thestate changes. Each change of state corresponds to an occurrence ofevent signal T. In the digital signal processing (DSP), two-bitcomparator 174 compares the value of two-bit counter 172 with previouscounter value register 176 during fourth cycle 136 of FIG. 2A. A changein the value of two-bit counter 172 indicated by the comparisoncorresponds to an occurrence of event signal T. Responsive to eventsignal T, digital gate 178 loads the new value of two-bit counter 172into previous counter value register 176. In this arrangement, the valuestored in two-bit counter 172 is not significant; rather, a change inthe counter value is detected.

With continuing reference to FIG. 5 and with further reference to FIG.5A, the polarity of waveform 34 along with an auxiliary “Misc2” signalare input to state machine 170 through “OR” gate 174. This arrangementenables the FPGA to generate event signals T for pulse welding and fora.c. welding. In the case of a.c. welding, Misc2 is set to zero so thatthe polarity signal feeds through to cycle counter state machine 170.For pulse welding, Misc2 is set to one when the arc is shorted, and zerowhen the arc is not shorted. FIG. 5A shows a graph of pulse current 180and the value of Misc2 182 when pulse welding is used instead of A.C.welding.

With continuing reference to FIG. 5 and with further reference to FIG.3, events initiated by an occurrence of event signal T are described. Atfourth cycle 136 of FIG. 2A, the digital signal processing performs acheck 190 to see if an occurrence of event signal T has been detected.This is done by comparing the current value of two-bit counter 172 withstored counter value 176 using two-bit comparator 174. If no change incounter value has occurred, the digital signal processing continues toloop through the four states 130, 132, 134, 136 of FIG. 2A. However, ifcheck 190 detects an occurrence of event signal T, the rms value iscomputed 192 as set forth in Equation (3) and in accordance with FIGS. 2and 7. Computation 192 is a background digital signal process.Additionally, a buffer switch 194 is performed so that whichever buffer(buffer A 70 or buffer B 72) had been active is switched to thebackground, and whichever buffer (buffer B 72 or buffer A 70) had beenthe background buffer is made the active accumulation buffer.

Exemplary digital signal processing circuitry and associated FPGAcircuitry for substantially real-time computation of rms voltageV_(rms), rms current I_(rms), and average power P_(avg) have beendescribed with reference to FIGS. 1-9. The described digital signalprocessing circuitry implements Equations (1) and (3) using Riemannsums, and is exemplary only. Those skilled in the art can readily modifythe illustrated digital circuitry or substitute other digital circuitryto perform these computations or substantial equivalents thereof. Theillustrated circuitry provides certain features that may be optionallyomitted or modified. For example, separate and independent digitalsignal processing pathways can be provided for computing each of the rmsvoltage V_(rms), rms current I_(rms), and average power P_(avg) values.In this arrangement, time-domain multiplexing aspects of the circuitrycan be omitted. Rather than having two alternating accumulators, asingle accumulator can be employed in conjunction with a storageregister that stores the previous sum for backgroundnormalization/square root processing. Moreover, if the digital signalprocessing is sufficiently fast or if parallel processing is employed,the temporary storage may be omitted entirely, and thenormalization/square root processing performed substantially in realtime for intervals between successive event signals T. Still further, atrapezoidal or otherwise-shaped integral element can be substituted forrectangular sample bars 122 of the Riemann sum illustrated in FIG. 4.Those skilled in the art can make other modifications to the exemplarydigital signal processing and FPGA circuitry illustrated herein forimplementing Equations (1) and (3) as digital circuitry.

With reference to FIG. 10, digital signal processing block 200 computesthe power factor (PF) in accordance with Equation (4) from the rmsvoltage V_(rms), rms current I_(rms), and average power P_(avg) values.The denominator of Equation (4) is computed using multiplier 202 actingon the rms current I_(rms) and rms voltage V_(rms) output by digitalsignal processing blocks 60, 60′ of FIGS. 7 and 8, respectively. Theaverage power P_(avg) output by digital signal processing bloc 60″ ofFIG. 9 is divided by this denominator using division block 204 tocompute the power factor PF.

With continuing reference to FIG. 10 and with further reference to FIG.11, electric arc welder 10 of FIG. 1 is readily adapted to implement aconstant power factor control of the weld process in pulse welding.Controller 30′ is a modified version of controller 30 of FIG. 1. Digitalerror amplifier 48′ produces error signal 50′ based on the power factorPF. Digital error amplifier 48′ compares the power factor PF output bydigital signal processing block 200 (shown in detail in FIG. 10) with PFset value 46′. Waveform generator 32′ modifies selected waveform shape210 based on error signal 50′ as described in Blankenship U.S. Pat. No.5,278,390 which is incorporated by reference herein.

With continuing reference to FIG. 10 and with further reference to FIG.12, electric arc welder 10 of FIG. 1 is similarly readily adapted toimplement a constant current welding process in which heat input to theweld is controlled by adjusting the power factor PF. Controller 30″ is amodified version of controller 30 of FIG. 1. The rms current 40 iscompared with set rms current 46 by digital error amplifier 48 toproduce current error signal 50 as in FIG. 1. Additionally, a seconddigital error amplifier 220 produces power factor error signal 222 bycomparing the power factor PF output by digital signal processing block200 (shown in detail in FIG. 10) with adjustable welding heat set value224. Waveform generator 32″ modifies selected waveform shape 210 basedon error signals 50, 222 as described in Blankenship U.S. Pat. No.5,278,390.

With reference returning to FIG. 11 and with further reference to FIG.13, in digital error amplifier 48′ the power factor error signaloptionally incorporates digital filtering. As shown in FIG. 13, digitalerror amplifier 48′ includes difference operator 232 that computesdifference signal 234 which is proportional to a difference between thecomputed power factor and power factor set value 46′. Difference value234 is input into digital filter 236 which generates control signal 50′for adjusting the waveform shape in accordance with the method describedin Blankenship U.S. Pat. No. 5,278,390. In one suitable embodiment,digital filter 236 is an infinite impulse response filter. The digitalfilter can be used to amplify the signal, smooth the signal, remove highfrequency signal components, or otherwise adjust the control signal.

With reference to FIG. 14, a digital error amplifier 240 for constantvoltage control is shown. Digital error amplifier 240 includesdifference operator 242 that computes difference signal E(n) 246 givenby:E(n)=V _(set)−(a·V _(avg) +b·V _(rms))  (6),where V_(set) is a set voltage value, V_(avg) is an average voltagevalue computed in accordance with Equation (5), a is an average voltageweighting factor implemented by multiplier 250, V_(rms) is the rmsvoltage of Equation (3) that is output by digital signal processingblock 60′ of FIG. 8, and b is an rms voltage weighting factorimplemented by multiplier 252. It will be recognized that differencesignal E(n) 246 can be biased by adjusting the weighting factors a and btoward average voltage control, rms voltage control, or a selectedweighted combination of average voltage and rms voltage control. Becausethe rms voltage is typically a better measure of the true heat input tothe weld by the welding process, the rms weight b is preferably greaterthan the average weight a, that is, b>a. Moreover, the sum of theweighting factors is preferably unity, that is, a+b=1. Optionally,difference signal E(n) 246 is processed by digital filter 254, such asan infinite impulse response filter, to amplify, smooth, or otherwisemanipulate difference signal E(n) 246 to produce control signal 256 foradjusting the waveform shape in accordance with the method described inBlankenship U.S. Pat. No. 5,278,390.

With reference to FIG. 15, a digital error amplifier 260 for constantcurrent control is shown. Digital error amplifier 260 includesdifference operator 262 that computes difference signal E(n) 266 givenby:E(n)=I _(set)−(a·I _(avg) +b·I _(rms))  (7),where I_(set) is a set current value, I_(avg) is an average currentvalue computed in accordance with Equation (5), a is an average currentweighting factor implemented by multiplier 270, I_(rms) is the rmscurrent of Equation (3) that is output by digital signal processingblock 60 of FIG. 7, and b is an rms current weighting factor implementedby multiplier 272. It will be recognized that difference signal E(n) 266can be biased by adjusting the weighting factors a and b toward averagecurrent control, rms current control, or a selected weighted combinationof average current and rms current control. Because the rms current istypically a better measure of the true heat input to the weld by thewelding process, the rms weight b is preferably greater than the averageweight a, that is, b>a. Moreover, the sum of the weighting factors ispreferably unity, that is, a+b=1. Optionally, difference signal E(n) 266is processed by digital filter 274, such as an infinite impulse responsefilter, to amplify, smooth, or otherwise manipulate difference signalE(n) 266 to produce control signal 276 for adjusting the waveform shapein accordance with the method described in Blankenship U.S. Pat. No.5,278,390.

With reference to FIG. 15A, an exemplary waveform shape adjustment inaccordance with the waveform shape adjustment method of Blankenship U.S.Pat. No. 5,278,390 is illustrated. Two waveforms 280, 282 are shown insolid and dashed lines, respectively. For b=1 and a=0 in Equation (6) orEquation (7) (for voltage control or current control, respectively),waveforms 280, 282 have equal rms values. However, the average value isgenerally different for waveforms 280, 282.Compared with waveform 280,waveform 282 has a reduced voltage or current background magnitude andan increased voltage or current magnitude in the pulse.

Moreover, it will be appreciated that the pulse repetition period ofwaveforms 280, 282 may be different. This difference in repetitionperiod is accounted for in the digital signal processing by performingthe Riemann sums of Equations (1), (3), and (5) over intervals betweensuccessive event signals T, instead of performing the Riemann summingover time intervals of fixed length. Generating event signals T at arising pulse edge or other identifiable characteristic of the waveformallows the summation interval to track the repetition period of thewaveform as the repetition period is adjusted by the waveform shaping.

With reference to FIG. 16, two digital error amplifiers 300, 302 computecurrent and voltage error signals for use in a constant current,constant voltage welding process control. Digital error amplifier 300includes difference operator 310, weighting factors a 312 and b 314, anddigital filter 316. Digital error amplifier 300 has the same voltageinputs and general circuit topology as amplifier 240 of FIG. 14;however, digital error amplifier 300 produces control signal 318 forcontrolling wire feed speed during the welding process. With increasingoutput of amplifier 300 the wire feed speed should be decreased, whilewith decreasing output of amplifier 300 the wire feed speed should beincreased. Digital amplifier 302 includes difference operator 330,weighting factors c 332 and d 334, and digital filter 336. Digital erroramplifier 302 has the same current inputs and general circuit topologyas amplifier 260 of FIG. 15, and produces control output 338 foradjusting the waveform shape in accordance with the method described inBlankenship U.S. Pat. No. 5,278,390. Hence, the waveform shape and thewire feed speed are simultaneously controlled using digital erroramplifiers 300, 302 to keep both voltage and current constant.

The invention has been described with reference to the preferredembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

1. An electric arc welder for performing a weld process with a selectedcurrent waveform performed between an electrode and a workpiece, saidwelder comprising: a sensor for acquiring measured values of a weldingparameter including measured welding voltage values; a digital signalprocessing block computing a voltage integration value of the measuredwelding voltage values integrated over a time interval; and an erroramplifier that derives a wire feed speed control signal for adjustingwire feed speed from the voltage integration value and a setpoint value.2. An electric arc welder as set forth in claim 1, wherein the erroramplifier comprises: a difference operator computing a differencebetween the voltage integration value and the setpoint value, the wirefeed speed control signal being derived from the computed difference. 3.An electric arc welder as set forth in claim 2 wherein the voltageintegration value is selected from a group consisting of (i) an averagevoltage value over the time interval, (ii) an rms voltage value over thetime interval, and (iii) a weighted sum of average voltage value overthe time interval and an rms voltage value over the time interval.
 4. Anelectric arc welder as set forth in claim 2 wherein the voltageintegration value comprises an average value of the measured weldingvoltage values over the time interval.
 5. An electric arc welder as setforth in claim 2 wherein the voltage integration value comprises aweighted sum of average voltage value over the time interval and an rmsvoltage value over the time interval.
 6. An electric arc welder forperforming a weld process with a selected current waveform performedbetween an electrode and a workpiece, said welder comprising: a sensorfor acquiring measured values of a welding parameter; a digital signalprocessing block computing an integration value of the measured valuesof the welding parameter integrated over a time interval, theintegration value comprising an rms value of the measured values of thewelding parameter over the time interval; and an error amplifier thatderives a wire feed speed control signal for adjusting wire feed speed,the error amplifier including a difference operator computing adifference between the integration value and the setpoint value, thewire feed speed control signal being derived from the computeddifference.
 7. An electric arc welder for performing a weld process witha selected current waveform performed between an electrode and aworkpiece, said welder comprising: a sensor for acquiring measuredvalues of a welding parameter; a digital signal processing blockcomputing an integration value of the measured values of the weldingparameter integrated over a time interval, the integration valuecomprising a combination of (i) an rms value of the measured values ofthe welding parameter over the time interval and (ii) an average valueof the measured values of the welding parameter over the time interval;and an error amplifier that derives a wire feed speed control signal foradjusting wire feed speed, the error amplifier including a differenceoperator computing a difference between the integration value and thesetpoint value, the wire feed speed control signal being derived fromthe computed difference.